1. Field of the Invention
The invention concerns a process for locating a fault on an optical transmission link using a coherent detection reflectometer in a terminal of the link. It also concerns fault location devices using this process.
2. Description of the Prior Art
Correct operation of wavelength division multiplex optical transmission networks requires it to be possible to test the essential components of such networks that the transmission links constitute.
This is particularly important if the links are amplified, as it must then be possible to assure that signals are transmitted correctly regardless of the wavelength concerned in the range of wavelengths used, given that a fault at a link amplifier can have different effects according to the wavelengths of the signals transmitted.
One prior art solution to the problem of verifying the correct operation of an optical transmission link consists in placing a coherent detection reflectometer in a terminal through which the link is able to receive at least one optical signal of particular wavelength in order, on the one hand, to send measuring pulses having a particular duration and a particular repetition rate over the link and, on the other hand, to locate any fault on the basis of the reflectometry signal obtained by means of a heterodyne receiver of the reflectometer. A loading signal at a wavelength different from that of the measuring pulses is conventionally associated with the measuring pulses to maintain a constant power level in the transmission link, when the latter is amplified, and consequently to maintain the quality of the reflectometry signal.
A solution of this kind cannot be used if filtering is employed, for example at the level of a network connection unit, to extract a signal of particular wavelength in order to transmit it via a branch link to a particular branch terminal. The loading signal is not diverted to the terminal with the measuring pulses between which is it interleaved.